This invention relates to impedance heating apparatus for heating the contents of tanks and vessels, and particularly tank cars such as railroad tank cars.
Railroad tank cars are often used to transport substances which are solid or very highly viscous at ambient temperatures. Such substances include coal tar pitch, asphalt and low-sulphur oil, among others. These substances must be heated in order to be pumped and transferred through pipes and also must be heated when stored in tanks so that they can be pumped out for further transporting or use.
In the instance of railroad tank cars, such substances are generally heated prior to being placed in the tank car and thus enter the tank car in a relatively liquid state. Despite providing the tank cars with thermal insulation, there are heat losses through the tank wall from the substance while it is in transit. Therefore, it is necessary to heat the substance while it is in the tank car, and particularly prior to pumping it out, which requires a relatively low viscosity. To this end, railroad tank cars have in the past been equipped with heating devices in the form of electrical resistance heating elements placed in tubes extending into the interior of the tank. Although the electrical resistance heater elements are capable of providing substantial amounts of heat within the tank, they have a number of drawbacks. Perhaps the most serious of these is the relative fragility of the electrical resistance heaters, whereby they often fail due to breakage from shock loads and vibrations involved in the use of railroad tank cars. Resistance heaters are all subject to fatigue failures or burnouts.
Also, by way of background of this invention, impedance heating of pipelines is known. In its simplest form, impedance heating comprises connecting a pipeline in a circuit to which alternating current source is applied, thereby utilizing the pipeline itself as a resistance heating element. The efficiency of impedance heating of pipelines is improved by positioning an insulated electrical cable parallel to the pipeline for completing the circuit through the pipeline. This sets up self-inductance between the pipeline and return cable, as well as utilizing hysteresis and eddy currents which would otherwise be lost, both of which increase the efficiency of the heating effect. A low voltage, e.g. 80 volts or less, and high amperage AC power supply is used, and the voltage appears on the exterior of the pipeline due to the electrical skin current conductive effect. Therefore, the heat is also generated on the outside of the pipeline, and is conducted through the pipeline to heat the substances flowing therethrough. The pipeline is generally surrounded by thermal insulation to prevent heat losses. The heat generated in the conductor itself is low, inasmuch as the conductor is normally copper or aluminum which has a low resistance and since the insulated conductor is outside the pipeline where any heat generated can dissipate rapidly.
A skin effect current tracing system has also been employed to heat pipelines, and particularly large pipelines where the skin of the pipe itself is too large to be used as the resistance in an impedance heating system. A small diameter heat generating pipe is welded to and "traces" the pipeline, and an insulated conductive cable is deployed on the interior of the smaller heat generating pipe, being electrically connected to the heat generating pipe at one end so that the heat generating pipe and insulated conductive cable are in series. An alternating current at relatively high voltage, e.g. 500-2,000 or more volts, may be applied, as required by the length of the heat generating pipe. The skin effect causes the current and voltage on the heat generating pipe to be on the interior surface thereof, with the exterior of the heat generating pipe thereby being at or near zero voltage for safety reasons. The heat generating pipe is heated by the current flow near the inside surface thereof, and the heat is thermally conducted to the larger pipeline. One of the most important limits on this type of heating system is the integrity of the insulation surrounding the conductive cable within the heat generating pipe, which breaks down at high temperatures. The insulation is also subject to damage when the cable is pulled into the heat generating pipe.
It will be apparent that large vessels, such as railroad tank cars, have surface areas which are too large to use the tank itself as the conductor in an impedance heating system. Further, the skin effect current tracing system can be employed by placing several heat generating pipes on the exterior of the tank, but there are obvious problems with trying to heat a large volume tank from the outside in, particularly in trying to insulate against heat losses and in the number of trace pipes required. Therefore, neither of these systems are wholly satisfactory for application to systems such as railroad tank cars.